With the increasing capacity of the new energy power plant, in order to achieve an optimal configuration of capacity of a grid-tied inverter, and to implement a solution of a megawatt system, parallel connection of inverters has become the trend of technical development. Parallel connection of AC sides of inverters has the following advantages.
A double split transformer at the AC side may be replaced by a two-winding transformer to reduce investment costs of the system, which retains original maximum power point tracking (MPPT), and has few modifications to the layout and structure of the system.
Referring to FIG. 1, a schematic diagram of inverters whose AC sides are connected in parallel in the conventional technology is shown.
PV1 to PVN are N PV arrays, that is an input terminal of each inverter is connected with an output terminal of a respective PV array, the first inverter to the Nth inverter are N inverters connected in parallel, the first filter to the Nth filter are N AC filters for the N inverters, and T is a two-winding transformer.
An AC bus is shared by the AC sides of the inverters, and DC sides of the inverters are connected to independent PV arrays respectively, thus N-way maximum power point tracking MPPT are performed, that is, MPPT of each inverter is performed independently.
Regardless of the advantages of the inverters whose AC sides are connected in parallel, the positive terminal and the negative terminal of the PV array have parasitic capacitances to ground, such that there is a coupling at a respective DC side of each inverter thereby forming a common-mode loop and causing a problem of common-mode circulation current. Referring to FIG. 2, a schematic diagram of a common-mode loop corresponding to the system shown in FIG. 1 is shown, where Cp/ntg1 is a parasitic capacitance of PV1 to ground, Cp/ntg2 is a parasitic capacitance of PV2 to ground. It can be seen that two inverters connected in parallel are coupled with each other through the parasitic capacitances to form a common-mode loop.
Therefore, it is a need for those skilled in the art to provide a control method and system for photovoltaic inverters whose AC sides connected in parallel, to restrain the problem of the common-mode loop for the system in which the AC sides of the inverters are connected in parallel.